1. Field of the Invention
The present invention relates to a super magnetostriction type actuator suitable for use in valves such as injection valves or shut off valves, and more particularly to a super magnetostriction type actuator in which changes in drive characteristics due to thermal expansion of a super magnetostriction rod can be minimized. For conciseness in the following the term super magnetostriction will be abbreviated to SMS.
2. Description of the Related Art
Generally known fuel injection valves utilizing a SMS type actuator such as those disclosed in publications of Unexamined Japanese Patent Publication No. 3-243174 and the like, comprise a cylindrical casing, a SMS rod for driving a drive object which is provided at one end of the casing, the SMS rod being housed inside the casing and extending in an axial direction thereof with one end attached to the drive object, and a solenoid coil disposed inside the casing around the SMS rod for applying a magnetic field to the SMS rod to cause an expansion or contraction thereof.
With a fuel injection valve involving this type of conventional technology, a valve seat incorporating a fuel injection port is provided at one end of the casing, with a drive object in the form of an internal opening type valve member seated thereon and urged in a normally closed direction by a valve spring. One end of the SMS rod is fixedly attached to the valve member so that when the SMS rod is contracted due to the magnetic field of the solenoid coil the valve is lifted against the resistance of the valve spring allowing fuel inside the casing to spray outwards.
The other end of the casing is provided with a cover member which abuts with an end face of the other end of the SMS rod. This cover member presses the SMS rod, together with the valve member, towards the valve seat and provides spring loading adjustment for the valve spring. Separation of the SMS rod from the cover when the rod is contracted due to the magnetic field of the coil is restrained so that the valve member can be positively lifted against the resistance of the valve spring.
With this conventional technology, the drive object in the form of a valve member is provided at one end of the casing, the cover is provided at the other end of the casing, and the SMS rod is located within the casing between the cover and the valve member. With this arrangement, when the solenoid coil provided around the SMS rod is heated by an external electrical load, the heating can cause the SMS rod to expand in an axial direction between the cover and the valve member.
If the coefficient of thermal expansion of the SMS rod is larger than that of the casing, then since the corner is fixed integral with the casing, when the SMS rod expands with heating it extends towards the valve member.
Consequently, with the conventional technology, not only is the spring load of the valve spring affected by the thermal expansion of the SMS rod, but also the lift amount of the valve member when the SMS rod is extended/contracted by the magnetic field from the solenoid valve is altered. This results in problems due to variations in injection flow rate characteristics with temperature.
Moreover, with a construction wherein the drive object is in the form of an externally opening type valve member which opens with extension of the SMS rod, the position of the valve member alters in the valve open direction with thermal expansion of the SMS rod, so that the valve member separates from the valve seat, causing problems with sealing of the valve.